Series editors' preface - Wood Tools

618 Conservation of Furniture
Acidic and alkaline stains Acids and alkalis
have been used as part of the traditional finishing
and restoration processes for centuries.
Wood is susceptible to damage by acid hydrolysis
and will be swollen by exposure to alkalis
(Mills and White, 1994). Health and safety data
sheets provided by the supplier should be
consulted to ensure appropriate precautions
are taken when handling these corrosive
materials. If diluting acids for workshop use,
gloves and goggles should always be worn.
Acid should always be added to water – the
reverse order can be dangerous.
Dilute sulphuric acid (H 2SO 4, 5–15%) was
traditionally used to treat a French polished
surface if excessive oil had been used. It can
also be used to neutralize iron stains on oak
and will impart a yellowish tone to many
woods.
Dilute nitric acid (HNO 3) was traditionally
used to treat ink stains (Hayward, 1988). A
25–35% solution can be brushed directly on to
bare wood, where it will oxidize the surface.
It can be used to reproduce the appearance
of mellow ageing on timbers such as satinwood,
rosewood, tulipwood and occasionally
walnut. Bennett (1990) recommends dampening
the surface of the wood with nitric acid
and then using a (pre-heated) hot air gun. The
gun is held a couple of inches away from one
end of the surface. After a few seconds a
colour change occurs, and this change can be
‘chased’ across the surface in a slow and
continuous movement. Care should be taken
not to expose adjacent materials to nitric acid
fumes as some, for example most metals, can
be severely damaged. Nitric acid can be difficult
to neutralize and any residues will attack
finishing materials applied over them. Some
restorers rinse and leave the surface for up to
a week before commencing any further
colouring work, whilst others use repeated
applications of a weak alkali such as sodium
bicarbonate. Treatment with nitric acid may be
followed by treatment with oxalic acid or
‘Superbleach’ (see above) to further chemically
modify the colour of bare wood.
Nitric acid vapour is particularly effective in
oxidizing organic molecules (Lee, 1967). The
fumes produced when a small piece of copper
or a copper alloy such as brass is placed into
a container of nitric acid have a marked
oxidizing effect on many woods and may be
particularly effective for matching aged satinwood.
Such fuming may be used to oxidize
replacement veneer or parts before they are
attached to the object, but must be carried out
in a sealed fume cupboard, because the fumes
are poisonous.
Dilute acetic acid (CH 3COOH, c.6%) was
traditionally used to treat ink stains (Hayward,
1988). Acetic acid can be combined with
ferrous material such as old nails or wire wool
to produce a stain traditionally used to colour
wood with a significant tannic acid content
(e.g. oak). The wire wool or nails are steeped
in a 30% solution of acid, agitated occasionally,
until a dark red brown solution is formed.
This is strained to remove large particles and
applied as a stain to bare wood. The iron in
the stain complexes with tannin in the wood
and gives a cold black coloration to the wood.
Oxalic acid (ethanedioic acid, HOOC⎯
COOH) is a weak dibasic organic acid that has
been used as a mild bleach and for removing
ink and iron stains in traditional furniture
restoration. The concentrations recommended
in finishing texts vary from saturated to 50/50
w/v solutions. It is brushed liberally onto the
surface and allowed to dry before being rinsed
from the surface. The dry crystals should not
be brushed or wiped from the surface as they
are an extreme irritant and are more liable to
be dispersed. Research has indicated that
residues of oxalic acid can remain within the
pores of wood, even after rinsing with
ammonium hydroxide. Such residues form
crystalline deposits of oxalic acid that have the
potential to damage both finishes applied over
them and the wood surface in which they are
lodged (German, 1989). Oxalic acid is a
poisonous substance and ingestion can be
fatal but the 5 g dose required is unlikely to
be consumed accidentally given its exceedingly
unpleasant taste.
Alkalis used to stain wood include sodium
hydroxide, ammonia and sodium carbonate.
The use of alkalis can cause some woods to
turn almost black whilst others, such as
mahogany, are reddened. Very dilute solutions
of sodium hydroxide (caustic soda, NaOH)
will impart an aged brown colour to cherry.
Sodium hydroxide was used in dilute solution
to darken or degrease wood. It is very corrosive
to skin and if splashed in the eyes can
cause corneal burns and blindness. It should